1. Field of the Invention:
The present invention relates generally to impedance matching devices for receivers and more specifically, to a device for presenting the active elements of a mixer with different RF and IF impedances to permit matching an antenna impedance to the optimum RF impedance of the mixer array while simultaneously matching the array's output IF impedance to an IF amplifier circuit.
2. Prior Art:
The impedance matching problem solved by the present invention is especially acute in SIS tunneling junction devices which have a typical IF impedance to RF impedance ratio of 12- However, most antenna and waveguide structures have a natural impedance of 100 Ohms or larger while the standard impedance for IF amplifiers is 50 Ohms and the optimum impedance for many low noise transistors is as low as 10 Ohms. The non-linear element in a mixer has its own optimum source (RF) and load (IF) impedance. The problem therefore is to build a network which matches the antenna impedance to the optimum RF impedance of the device and also matches the device's output IF impedance to the IF amplifier circuit. For typical Schottky diodes with a local oscillator signal applied, the optimum RF source and the IF impedance are usually close to each other. In this case, the matching requirements are modest and standard techniques such as quarterwave transformers are satisfactory for accomplishing the conversion requirement despite their sometimes bulky configuration for use in mixer arrays. Accordingly, while the present invention has application in virtually any impedance matching circuit, the principal advantage resides in its small size and high conversion ratio especially useful in conjunction with SIS tunnel junction devices and it is in conjunction with such devices that the particular embodiment of the invention herein disclosed relates.
The current principal application for receiving devices which utilizes SIS tunnel junction devices is in the radio astronomy art. SIS tunnel junction devices have a very high sensitivity and operate extremely well in the millimeter and sub-millimeter wavelength ranges of radio astronomy. In addition, SIS tunnel junction devices are extremely small making them physically compatible with the millimeter and sub-millimeter wavelengths of radio astronomy. An article describing the use of the SIS tunnel junction devices in radio astronomy and which includes a detailed description of the tunnel junction device itself, is entitled "Super Conducting Tunnel Detectors In Radio Astronomy" by Thomas G. Phillips and David B. Rutledge published in Scientific American in the May 1986 edition (Volume 254, No. 5) at pages 96-102. Other references of some relevance to the present invention include; An article entitled "Negative Resistance and Conversion Gain In SIS Mixers" by A. D. Smith, et al published in Physica 108B, pages 1367-1368 (1981) by North-Holland Publishing Company. An article entitled "Large Gain, Negative Resistance and Oscillations In Superconducting Quasiparticle Hetrodyne Mixers" by W. R. McGrath et al from Applied Physics Letters 39(8), 15 Oct. 1981, pages 655-658; an article entitled "Broad-Band RF Match To a Millimeter-Wave SIS Quasi-Particle Mixer" by W. R. McGrath et al, IEEE Transactions On Microwave Theory and Techniques, Volume MTT-33, No. 12, Dec. 1985, pages 1495-1500; an article entitled "Integrated Tuning Elements For SIS Mixers" by A. R. Kerr et al, published at the 1987 International Superconductivity Electronics Conference; an article entitled "SIS Mixer to HEMT Amplifier Optimum Coupling Network" by S. Weinreb, published in the IEEE Transactions On Microwave Theory and Techniques, Volume MTT-35, No. 11, Nov. 1987, pages 1067-1069; and an article entitled "Noise Parameters of SIS Mixers"by L. Addario, submitted to the IEEE Transactions On Microwave Theory and Techniques dated Sep. 23, 1987. The applicant herein knows of no prior art impedance matching devices which may be packaged in as small a space as the present invention nor which are capable of matching impedances having such a high ratio with respect to each other.